Fix infinite loop bug in X² CDF

[omaus]

Please reference the issue(s) this PR is related to

Fixes #217

Please list the changes introduced in this PR

• Bug fixed when using ChiSquared.CDF with arbitrary dof and x being nan

Description

When using the CDF in a ChiSquared distribution with input value nan, Gamma.lowerIncomplete resp. gcf goes into infinite loop.
CDF function now checks for input being nan and returning nan if so.

[Required] please make sure you checked that

• [×] The project builds without problems on your machine (and bugfix works, too)

[Optional]

• Added unit tests regarding the added features (will be done in near future)

[codecov-commenter]

Codecov Report

Merging #218 (b83b8a7) into developer (262f1ac) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           developer     #218   +/-   ##
==========================================
  Coverage      31.61%   31.62%           
==========================================
  Files            124      124           
  Lines          11664    11658    -6     
  Branches        1784     1785    +1     
==========================================
- Hits            3688     3687    -1     
+ Misses          7535     7529    -6     
- Partials         441      442    +1     

Impacted Files	Coverage Δ
src/FSharp.Stats/Distributions/Continuous.fs	15.85% <0.00%> (-0.04%)	⬇️
src/FSharp.Stats/Fitting/Spline.fs	91.17% <0.00%> (-0.26%)	⬇️
src/FSharp.Stats/Array.fs	0.00% <0.00%> (ø)
...tats/ML/Unsupervised/PrincipalComponentAnalysis.fs	0.00% <0.00%> (ø)
src/FSharp.Stats/Ops.fs	4.00% <0.00%> (+0.15%)	⬆️

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